The invention relates to a process for producing a fried starch-containing foodstuff, such as fried potato, fried vegetables or fried fruit, having improved crispness.
U.S. Pat. No. 4,751,093 describes a process of preparing fried potato pieces with improved texture and improved texture stability, wherein the potato pieces are disrupted at discrete locations of the surface to expose the interior of the pieces so as to allow moisture to escape during finish frying. The disruption is effected by scoring with a knife, after drying and partial fiying (par-frying) and before finish flying. However, this treatment drastically changes the appearance of the product.
U.S. Pat. No. 5,141,759 discloses a potato product with increased crispness, which is produced by coating washed, cut, blanched and dried potatoes with an aqueous starch slurry and then par-frying the potato strips and freezing them. The starch slurry consists of chemically modified ungelatinized potato starch, water and other components such as corn starch and rice flour. U.S. Pat. No. 4,456,624 is aimed at enhancing crispness by impinging a high velocity stream of food particles such as potato granules, flakes, starch, flour and similar potato components, against the strips. However, the processes according to U.S. Pat. No. 5,141,759 and U.S. Pat. No. 4,456,624 require additional processing steps and additional ingredients in order to prepare and fixxe2x80x94by means of coating or impactingxe2x80x94a separate food component.
According to GB-A-2046070, blanched strips are subjected to a two-stage drying process, in which they are first exposed to high velocity ambient air and then further dried in circulating heated air. After subsequent par-frying, freezing and finish-frying, the product should demonstrate a mealy core and remain crisp and rigid for extended periods. The total moisture loss over both drying steps is about 18-26% by weight U.S. Pat. Nos. 3,397,993 and 3,649,305 also disclose hot air drying procedures of blanched (""993) and non-blanched (""305) potato strips, resulting in similar levels of moisture removal (20-30% and 10-30%, respectively). Texturing of French fries by surface-freezing to control crispness and rigidity is disclosed in Food Technology (1972) 61-68. GB-A-2157539 relates to making potato boats involving abrading the inner potato surface with starch particles. DE-C-738841 is concerned with producing fluffy potato pastry involving blanching, drying and puffing without flying.
Although these prior art products may have an increased crispness immediately after the production process, the crispness after finish frying and after a holding time of 10-20 minutes is still unsatisfactory. Consequently, there is a need for a simpler and more effective method of producing fried foodstuffs having an improved crispness, even a considerable period of time after finish frying.
It has been found now that this need can be fulfilled by a process comprising a thermal treatment wherein starch is gelatinized, optional drying, partial frying, cooling optional freezing and optional finishing the foodstuff, which is characterized in that one or more surface planes of the foodstuff is subjected to a mechanical surface treatment comprising a pressure drop at elevated temperature between said thermal treatment and said partial frying.
The mechanical treatment according to the invention is aimed at physically modifying essentially the entire surface area of one or more surface planes an is an essential step in the process of the invention. It is not necessary that the entire surface area of the foodstuff planes is modified and not even the major part of it, in order to improve crispness and especially retention of crispness of the finished product to a considerable extent. Since the method of the invention envisages to modify only the surface of the foodstuff, the depth of the modification within the product will be confined to the peripheral layers of cells. Therefore the modification will result in strips which are similar in appearance to conventionally produced fried foodstuff, such as potato chips. An additional and unexpected advantage of the present process is that the product obtained requires a shorter finish-fiying time.
The term xe2x80x9cfinishingxe2x80x9d as used herein denotes any final heat treatment for obtaining a ready-to-eat foodstuff. It will denote finish flying, i.e. cooking in oil or fat, but it can also mean other heat treatments such as oven baking (hot air, grill, microwave etc.). The effects of the treatment of the invention occur irrespective of the type and the timing of the finishing treatment.
The process according to the invention can be applied to any starch-containing foodstuff, such as root crops, tuberous plants such as potato and taro, and starch-containing fruits such as bananas. The process can also be used for fried particulate products largely consisting of starch, such as rice, tapioca, manioc an the like. The process is especially suitable for producing fried potato products.
The process according to the invention is described below in more detail with a reference to fried potato chips (American: French fries), but it should be understood that the process is equally applicable to other foodstuffs as defined here above, including potato crisps (referred to in some languages including German and Dutch as xe2x80x9cchipsxe2x80x9d).
Potatoes destined for the production of chips are first steam-peeled by quickly cooking the skin at high pressure and removing it by brushing, rubbing or spraying. Then the potatoes are selected to remove blemishes, and are cut according to the desired final shape. The cut potatoes are then blanched, which involves heating at 65-100xc2x0 C. for a short time so as to gelatinize the starch and to leach out soluble reducing sugars. Blanching may be done repeatedly, with intermittent cooling. After optional dipping (addition of chemicals such as calcium or sugars) the blanched potato pieces are usually partially dried at 60-140xc2x0 C., in order to enhance crispness, to reduce fat uptake and/or to save energy.
The next step in potato chip production is partial deep-frying (par-frying) for 30 seconds to 6 minutes at 135-190xc2x0 C. in cooling oil or fat in order to evaporate water at the surface. After par-frying, superficial fat may be removed and then the chips are cooled and, in case of frozen chips, deep-frozen. Before consumption, the chips are finish fried.
The mechanical surface treatment according to the invention results in damaging the surface structure especially of the surface cells of the gelatinized foodstuff. Such treatments include xe2x80x9cexternalxe2x80x9d treatments such as sanding, brushing, rubbing, and the like, but also treatments which induce xe2x80x9cdamaging from insidexe2x80x9d, e.g. expansion by forced evaporation, such as sudden pressure drop (steam, air or the like) at high temperatures irradiation. The mechanical surface treatments is preferably carried out above 0xc2x0 C., i.e. without freezing, and does not just consist of a liquid treatment. Preferred temperatures for the mechanical treatment are between 40xc2x0 and 1000xc2x0 C., preferably between 55xc2x0 and 90xc2x0 C. The mechanical treatment may be performed using conventional equipment, such as carborundum sanding apparatus or steam peeling apparatus.
The pressure drop comprises a drop of at least 2.5 bar, preferably at least 5 bar, up to 200 bar. The initial pressure before the pressure drop is preferably at least 5 bar, up to about 200 bar. The lower pressure after the pressure drop is preferably 1 bar or lower, down to vacuum (lower than 0.01 bar). The depressurizing time is less between 2.5 seconds and 10 minutes, preferably between 10 seconds and one minute, typically 20-30 seconds; the depressurizing rate is advantageously at least 0.5 bar per second up to 5 bar per second. The pressure drop is preferably applied at a water content of the foodstuff of between 35% and 90%, most preferably between 50% and 85%, typically around 80%.
The mechanical treatments will result in a maximum enhancement of crispness and crispness retention in the finished product, when the starch in the peripheral layers of cells has been previously gelatinized. This can be done by blanching the raw tissue prior to the external mechanical treatment. It should be understood that the term xe2x80x9cblanchingxe2x80x9d as used herein comprises any treatment involving exposure of the surface layer of the product to temperatures above 55xc2x0 C., so as to allow gelatinization of the starch. If the mechanical treatment already comprises conditions resulting in surface gelatinization, such as with steam pressurizing-depressurizing, a previous blanching step is not required. The raw tissue can, immediately after washing, peeling and/or cutting, be subjected to a treatment under elevated (steam) pressure and a subsequent sudden pressure drop. Preferably such treatment is immediately followed by a drying and/or frying step, in order to fix the physically modified surface.
If a drying step is performed in the food production process, then the external mechanical treatment is preferably inserted between the blanching step and the drying step. The partial drying can be effected by air-drying for several minutes at temperatures of e.g. 60-140xc2x0 C. The drying treatment may result in a fixing of the outer structure of the product as obtained by the surface treatment. It was found furthermore, that freezing the treated material is not necessary for obtaining an optimum crispness.
The product obtained by the process of the invention has an improved crispness, and remains crispy for an extended period of time, e.g. longer than 10 minutes after finish flying. At first, sight, the resulting product does not differ from conventionally produced chips, wherein the surface structure has not been disrupted after blanching. A closer examination, however, shows that the crust of the product has a fine-grained and crumbly-like appearance. A crispy and crackle-like mouthfeel is experienced upon eating. The product also allows a faster finish-frying.